Perhaps a more acceptable and open source friendly way to do this would be to check to see if the gem's source is available on something like github. Then create a fork of the gem code, make your changes, and send a pull request. Chances are, someone else wants that functionality too and you will be heralded for your contribution.

Newer versions of bundler allow you to do this with the following command:

bundle open gem_to_edit

This will open the gem code in your default Text editor.

(Hint: if you are on Mac OSX/Linux, you can specify your default text editor by changing the $EDITOR variable. Say you are on bash, you could just open your ~/.bash_profile and add: export EDITOR='subl -w' to set up Sublime Text 2 as your default editor)